The present invention relates to a method for calibrating a measurement device by means of a target object as well as a measurement device for performing such a method.
The term “calibrate” defines a measurement procedure in which the deviation between a first and a second dimension is determined and documented. The determined deviation is taken into account in the subsequent use of the measurement device for correction purposes.
Optical measurement devices comprise one or more laser measurement apparatuses, a camera device, and a control device, wherein the laser measurement apparatuses are designed as angle measuring devices or distance measuring devices. By means of optical measuring devices, angles, distances, and positions of target objects can be determined. The term “target object” includes all objects that reflect, scatter or reflect and scatter the striking laser beam.
When measuring natural targets, such as corners or edges of buildings, the measurement accuracy of the measuring devices depends largely on how exactly the target axis of the camera device and the measurement axis of the laser measuring device are arranged coaxially. The coaxial deviations of the measurement and target axis increase, the more moveable optical components a measurement device has. Camera devices with zoom lenses have a high optical resolution, yet simultaneously have an unstable target axis.
It is known for operators to check the coaxiality of the measurement and target axis of a measuring device, and if necessary, to manually correct it by means of adjusting elements. The manual adjustment by the operator is time-consuming.
The object of the present invention consists of developing an automatic method for calibrating a measurement device that can be performed without an operator intervening. In addition, a corresponding measuring device is to be developed for performing the calibration procedure.
In the method already mentioned for calibrating a measurement device according to the invention, this object is achieved by the features of the independent claim. Advantageous embodiments are indicated in the dependent claims.
According to the invention, the method for calibrating a measuring device, which has a laser measurement apparatus that emits a laser beam along a measurement axis, a camera device with a target axis, and a control device, comprises the following steps:                Taking a first image of a target object by means of the camera device, with the laser beam turned off,        Defining an image section around the target axis position, stored in the control device, in the first picture of the target object,        Determining a first contrast between the darkest image region of the image section and the brightest image region of the image section,        Comparing the first contrast against a first limit value stored in the control device,        Taking a second image of the target object, with the laser beam turned on, by means of the camera device, if the first contrast does not exceed the first limit value,        Determining a differential image between the first and second image of the target object,        Identifying a laser beam and a surrounding region in the differential image of the target object,        Determining a second contrast between the brightest image region of the surrounding region and the darkest image region of the laser beam,        Comparing the second contrast against a second limit value stored in the control device,        Determining a focal point of the laser beam and the focal point coordinates, if the second contrast does not fall below the second limit value, and        Storing the focus point coordinates of the laser beam as a new position of the target axis in the control device.        
The method according to the invention has the advantage that the calibration of the measuring device is performed by means of the target object only when the target object meets two evaluation criteria. As the first evaluation criterion, the uniformity of the brightness of the target object is inspected by means of the first contrast, and as the second evaluation criterion, the difference in the brightness between the identified laser beam and the surrounding target objet is inspected using the second contrast.
The images of the target object are evaluated by means of known image processing and object identification procedures. The term “object identification” includes procedures for identifying a known object within an object space by means of optical, acoustic, or other physical detection procedures.
The contrast is a differentiating feature for the brightness trend of an image or the brightness difference between two image regions. As a physical measurement parameter for brightness, one uses the luminous intensity that is measured in candela or a comparable measurement parameter. For every pixel of the image sensor, the camera device provides a pixel value. The image region refers to the smallest unit of the image of the target object that is used for evaluating the target object within the scope of the method according to the invention. The image region may comprise a single pixel or multiple neighboring pixels. If the image regions each comprise one pixel, the pixel values of the individual pixels are compared in calculating the contrast; for image regions with multiple pixels, the pixel values are averaged into a measurement value and the averaged measurement values are subsequently compared.
The first contrast is defined by the difference between 1 and the ratio of the measurement values between the darkest image region (minimum measurement value) and the brightest image region (maximum measurement value) of the image section and is indicated as a percentage value. To limit the measurement error during calibration, the target object should have the most uniform surface structure possible, which is represented as uniform brightness in the image of the target object. The deviation of the image regions to the brightest image region of the image section must be less than a first limit value. The first limit value is for example 5%, i.e., all image regions of the image section have a deviation less than 5% in relation to the brightest image region. The first limit value is established beforehand and is stored in the control device.
After comparing the first contrast against the first limit value, the calibration procedure is discontinued if the first contrast exceeds the first limit value; the calibration procedure is only continued if the first contrast fails below, or does not exceed, the first limit value. If the first contrast of the image section exceeds the first limit value, the target object is not suited for calibrating the measurement device and the calibration procedure is discontinued. The measurement device may be switched by the control device from a calibration mode into a measurement mode; for additional measurements, one uses the position stored in the control device as the position of the target axis.
The second contrast is defined by the difference between 1 and the ratio of the measurement value between the brightest image region (maximum measurement value) of the surrounding region and the darkest image region (minimum measurement value) of the laser beam, and is indicated as a percentage value. To limit the measurement error during calibration, the laser beam should have a greater brightness in relation to the surrounding region. The difference between the image regions of the laser beam and the image regions of the surrounding region must be greater than a second limit value. The second limit value amounts to 10% for example, i.e., all image regions of the laser beam have at least a 10% higher luminous intensity than the image regions of the surrounding region. The second limit value is established beforehand and stored in the control device.
After comparing the second contrast against the second limit value, the calibration procedure is discontinued if the second contrast falls below the second limit value; the calibration procedure is only continued if the second contrast exceeds, or does not fall below, the second limit value. If the second contrast falls below the second limit value, the target object is unsuited for calibrating the measurement device and the calibration procedure is discontinued. The measurement device may be switched by the control device from a calibration mode into a measurement mode; for additional measurements, one uses the position stored in the control device as the position of the target axis.
A preferred development of the method according to the invention for calibrating a measurement device is characterized by the additional steps:                Performing a distance measurement to the target object and        Comparing the measured distance against a distance range stored in the control device.        
The distance of the measurement device to the target object is an additional evaluation criterion that evaluates the quality of a target object for calibrating a measurement device. The distance measurement and the comparison against the distance range stored in the control device may be performed before the camera device takes the first image of the target object, after the first evaluation criterion was inspected, or after the second evaluation criterion was inspected.
In a particularly preferred manner, the calibration procedure is discontinued when the measured distance of the measurement device to the target object lies outside the stored distance range; the calibration procedure is only continued when the measured distance lies within the stored distance range. To limit the measurement error during calibration, the target object should be arranged within an established distance range. If the distance lies outside the distance range, the target object is unsuited for calibrating the measurement device and the calibration procedure is discontinued. For distances that lie above the distance range, there is the risk that the luminous intensity of the image regions within the laser beam is too low. The measurement device may be switched by the control device from a calibration mode into a measuring mode; for additional measurements, one uses the position stored in the control device as the position of the target axis.
To perform the procedure according to the invention for calibrating a measurement device, the measurement device comprises:                A laser measurement apparatus that emits a laser beam along a measurement axis,        A camera device with a target axis, and        A control device with a control element for controlling the laser device and the camera device, an evaluation element, and a storage element.        
In a preferred development of the measurement device, the camera device has a camera lens, which is adjustable between a first and second focal length, wherein the storage element stores for the first focal length of the camera lens a first position of the target axis and said storage element stores for the second focal length of the camera lens a second position of the target axis. In a camera lens system with moveable lenses, the alignment of the target axis changes by moving the lenses. The measurement accuracy of a measurement device can be improved if the respective position of the target axis is stored for various focal lengths of the camera lens system.
In a particularly preferred manner, the camera lens system can be switched between more than two focal lengths, wherein the storage element stores a position of the target axis for every focal length of the camera lens system. The instability of the camera lens system and thus the instability of the target axis increase, the larger the mechanical adjustment range is for moveable lenses. The measurement accuracy of a measurement device can be improved if the respective position of the target axis is stored for various focal lengths.
Embodiments of the invention are hereinafter described using the drawing. It is intended to depict the embodiments not necessarily to scale; instead, the drawing, where helpful for explanation purposes, is executed in a schematic and/or slightly distorted form. In regard to amendments of teachings directly recognizable from the drawing, one shall refer to the relevant prior art. One shall thereby take into account that various modifications and changes regarding the form and detail of an embodiment may be undertaken, without deviating from the general idea of the invention. The features of the invention disclosed in the description, the drawing, as well as the claims may be essential individually per se as well as in any combination for the development of the invention. In addition, the scope of the invention includes all combinations of at least two features disclosed in the description, the drawing and/or the claims. The general idea of the invention is not restricted to the exact form or the detail of the preferred embodiment depicted and described below, or limited to a subject matter that would be restricted in comparison to the subject matter claimed in the claims. In regard to the given measurement ranges, values lying within the mentioned limits are to be disclosed as limit values, and are to be used and claimed as desired. For simplicity's sake, the same reference signs are used below for identical or similar parts, or for parts with an identical or similar function.